A VCSEL is a semiconductor laser that emits light in a vertical direction with respect to a substrate. In general VCSEL have an active layer with a large gain, a low threshold current, high optical power, reliability, and adequately controlled polarization. Since the VCSEL does not require a cleavage process, it allows to be integrated into two-dimensional arrays for on-wafer testing. It is suitably used in various consumer applications such as the light source of an image forming apparatus, the light source of an optical pickup device, the optical communication data transmitter of optical interconnections and optical modules, etc.
Optical modules made with VCSELs have potential applications in high-speed transmission of light. Generally, the optical power, optical spectrum, and end of life time of a semiconductor laser are affected by the degree of heat generation resulting from current injection. It is particularly noted that since the VCSEL has its active layer arranged between two semiconductor multilayer reflector (DBR: Distributed Bragg Reflector) mirrors that have high thermal resistance, significant temperature increase may occur in the active layer, and due to this higher junction temperature of the VCSELs in comparison to the optical module heat generated components like CDR etc, the performance of VCSEL will be seriously degraded.
Accordingly, techniques have been proposed for controlling such temperature increase in the active layer of the VCSEL. US Patent Publication No. 2005/0271113 A1 discloses a VCSEL 100 as shown in FIG. 1. The VCSEL 100 includes a top DBR 120 and a bottom DBR 130 sandwiching an active region 140. The top and bottom DBRs 120, 130 and the active region 140 are fabricated on a substrate 150. A top electrode 154 is connected to the top reflector 120 while a bottom electrode 152 is connected to the substrate. The active region 140 includes a light generation layer 142 which is typically constructed from one or more quantum wells of In Gallium Arsenide (GaAs), Aluminum Arsenide (AlAs), Aluminum Gallium Arsenide (AlGaAs), or Indium Aluminum Gallium Arsenide (InAlGaAs). The light generation layer 142 is configured to generate light having a known wavelength. When the current is applied to the electrodes 152, 154, then flows through the active region 140, photons are generated by the quantum wells of the light generation layer 142. Light is reflected back and forth between the DBRs 120 and 130, finally a portion light is transmitted through the DBR 120 and out of a contact window 156 in the top electrical contact, which is indicated by arrow 158.
To generate more light from the VCSEL 100, more current is applied to the VCSEL 100, which brings more heat generated at the active region 140. Accordingly, to facilitate dissipation of heat produced by the active region 140, a high thermal conductivity layer 102 is fabricated between the active region 140 and the bottom DBR 130. Particularly, the high thermal conductivity layer 102 is made of AlAs, which has a high optical transparency and has relatively higher thermal conductivity compared to the thermal conductivity of the DBRs 120 and 130. Therefore the high thermal conductivity layer 102 facilities the removal of and dissipation of heat from the active region 140.
However, when the VCSEL 100 is operated with high current for long durations, it's hard to dissipate all heat generated by the active region 140 and as a result, high junction temperature may cause unstable operation of the VCSEL 100 at higher ambient temperature, which reduces the performance of the VCSEL 100. On the other hand, a portion of heat also will transmit to the top DBR 120 and heat will be present on the electrodes 154 due to the injected current herein, which is congregated on the top side of the VCSEL 100, especially on the top surface of the top DBR 120.
Thus, it is desired to provide an improved VCSEL with high heat dissipation efficiency, and manufacturing method thereof to overcome the above-mentioned drawbacks.